214 Section 3 Ferrous Metallurgy As discussed earlier in the chapter, cooling curves A and B in Figure 10-21, soaked at 1300°F (700°C) and 1000°F (540°C), produce coarse and fine pearlite, respectively. Rapid Cool and Intermediate Soak—Bainite Samples of G10800 steel cooled rapidly along cooling curve C in Figure 10-21 form bainite. Bainite is a microstructu re in steel with much smaller platelets than pearlite, Figure 10-22. Any cooling curve that misses the pearlite nose and holds between 600°F and 900°F (320°C and 480°C) will produce a bainite microstructure. An alloy with a bainite structure has a much higher yield strength and tensile strength than the same alloy with a pearlite structure. It also has lower elongation and less formability. Processing requires more care than simply cooling the workpiece. The parts must be cooled quickly from above the A1 temperature to between 600°F and 900°F (320°C and 480°C) in less than one second, but then the part must be held at the soak temperature for about an hour before cooling to room temperature. Interrupting the cooling step at the intermediate temperature puts less “cooling stress” on the workpiece due to dimensional changes during cooling, so parts are less likely to crack in bainite processing. Some gear shafts and alignment pins require steel with very high strength and hardness, and bainite may be specified for these applications. Processing procedures and equipment used to develop a bainite microstructure are discussed in Chapters 11 and 12. 1 200 600 1000 1400 °F 10 100 Cooling time (seconds) 1 minute A1 transformation temp Cooling curve (A) Cooling curve (B) Cooling curve (C) Cooling curve (D) 1 hour 1 day 1 week 1,000 10,000 100,000 Goodheart-Willcox Publisher Figure 10-21. The time-temperature transformation path taken by four samples are marked as cooling lines A, B, C, and D. Path A, produced with a salt bath at 1300°F (700°C), results in coarse pearlite. Path B, with a salt bath at 1000°F (540°C), results in fine pearlite. The extremely rapid cooling of paths C and D do not allow pearlite formation, but instead lead to different structures. ASM International Figure 10-22. A UNS G10800 sample cooled rapidly from 1500°F to 700°F (820°C to 370°C) forms a structure called bainite. The dark particles, cementite, are small, short platelets instead of the larger, continuous, interleaved platelets found in pearlite. 30 μm Copyright Goodheart-Willcox Co., Inc.